Conventionally, there has been proposed a spark plug in which, in order to improve ignition performance and durability of its ground electrode, a noble metal tip is embedded into the ground electrode by means of resistance welding such that the noble metal tip projects from the distal end of the base member of the ground electrode (see, for example, Japanese Patent Application Laid-Open (kokai) No. 2009-129908, “Patent Document 1”). In the case of the ground electrode in which a noble metal tip is embedded into the ground electrode base member, due to heat generated in an internal combustion engine, oxide scale is formed at a joint portion between the ground electrode base member and the noble metal tip in some cases. Excessive formation of such oxide scale may result in separation of the noble metal tip from the ground electrode base member.